An expired domain that
completely goes through the entire expiration process at the registrar AND registry and then gets completely dropped from the registry database (typically around 84 days after expiration, which can vary quite a bit depending on the TLD) will never retain it's "age". Effectively, once dropped from the registry database, the domain no longer exists, and when it is hand registered or drop-caught, it becomes a "new" entry in the registry database. There is
no way around this.
That being said .. you can still acquire expired domains and retain their age .. but you need to acquire them before they leave the registrar portion of expiration (typically about 42 days after expiration depending on the TLD). Not all domains are available in this phase, but a majority of them are via auction. It depends on what registrar they were hosted at when they expired (for example, domains that expire at GoDaddy and many other registrars can be found at GoDaddy Auctions).
GoDaddy auctions their expired domains on Day 35 after expiration, so if someone wins it at auction (or buys it on days 36 to 40 at closeout), then instead of GoDaddy sending the expiring domain to the registry (who hold the expired domain in a blackout period of about another 42 days), GoDaddy sends it over to your account and the domain is never dropped from the registry database .. and as such keeps it's original registration data (age).
Take a look in my signature or at NameCult as every day I list hundreds of the best domains that are going to auction that day, as well as tons of domains that passed the day 35 auction and are now in "closeout", where instead of bidding against other people, you can simply buy the domains for $5 to $11.
I personally acquire over 95% of my domains via GoDaddy auctions and closeout.
Do a search for domain life-cycle .. you'll get a ton of results explaining the entire expiration process!